1. Field of the Invention
The present invention relates to a sheet feeding device.
2. Description of the Related Art
A commonly-used image forming apparatus, such as a printer, capable of processing a plurality of sheets includes a sheet feeding device. Such an image forming apparatus separates sheets individually from a stack of sheets and sends them one by one to a printing unit. Thus, the stack of sheets can be individually processed and printed. If, however, a sheet is caught somewhere on its transport path and not be transported, a transport error, i.e., feed jam, occurs. This feed jam causes a jam of the subsequent sheets at a location where the feed has occurred. The jammed sheets may all become wasted, or may be sent forward in bundle and result in machinery damage.
Some of those including a sheet feeding device have a mechanism for detecting a transport error. For example, Japanese Patent Application Laid-open No. Heisei 6-191018 discloses a conventional feed-jam detecting device including a feeding sensor that detects a sheet separated from a stack of sheets and transported to an area immediately before a printing unit. The conventional feed-jam detecting device includes a timing switch and a counter that measure elapsed time from the separation of a sheet until the feeding sensor detects the sheet. The conventional feed-jam detecting device also includes a stop-controller that stops feeding of sheets when the feeding sensor does not detect a sheet after an elapse of a predetermined time from the separation of the sheet.
Thus, feeding of sheets stops when the feeding sensor does not detect a sheet because of a feed jam, the number of waste sheets caused by a feed jam can be reduced. Besides, sheets are prevented from being sent forward in bundle, does not cause machinery damage.
As described above, conventional sheet feeding devices include detect a feed jam by a detecting unit, such as the feeding sensor, that detects a sheet transported to a predetermined location. However, a feed jam may not be accurately detected because the occurrence of a feed jam is determined based only on the feeding sensor and the elapsed time. The above conventional feed-jam detecting device measures elapsed time from the separation of a sheet until the feeding sensor detects the sheet, and compares the elapsed time and a predetermined time, i.e., a time expected to be taken, set in advance. Therefore, for example, when a transport speed of a sheet decreases for some reason and the elapsed time unit the feeding sensor detects the sheet exceeds the predetermined time, a feed jam is determined to have occurred even when such a feed jam has not occurred.
On the other hand, when the moving distance is detected and whether the sheet has reached the feeding sensor is judged based on the detected moving distance and a distance to the feeding sensor, whether the feed jam has occurred can be detected more accurately. However, depending on circumstances under which the sheet is being transported, the sheet may move in a direction opposite to a normal moving direction during transport. In this case, an accurate moving distance may not be detected.